Coated near-alpha titanium articles

ABSTRACT

Articles of titanium based materials are protected against alpha case formation such as results from exposure to air at high temperatures by coating with an iron-chromium based alloy. The coating may be deposited by ion plating, sputter plating or sputter ion plating these methods being preferred for avoidance of surface contamination. A preferred iron to chromium ratio (by weight) is 10:1 to 2.5:1. Specific coating compositions (given by weight) are: (a) 12Cr-1(Mo+Zr)-0.15C-balance Fe, b) 15.5Cr-4.8A1-0.3Si-0.03C-balance Fe.

This invention relates to protective coatings for articles of near -αtitanium based alloys. It is particularly concerned with coatings forgas turbine engine compressor stage components formed in near -αtitanium based alloys suitable to convey at least high temperatureoxidation resistance and preferably also high temperature corrosionresistance to these components.

Pure titanium undergoes an allotropic transformation at 882° C. from thelower temperature phase, designated the α phase, which has a hexagonalclose-packed (hcp) structure to a higher temperature phase, designatedthe β phase, which is stable up to the melting temperature and had abody centre cubic structure. Titanium alloys are conventionallycategorised as α-type alloys, β-type alloys or α+β alloys by virtue ofthe nature and level of the alloying additions they contain. Near -αtitanium alloys are titanium based with additions of, amongst otherthings, α-stabilising elements such as aluminium and tin that promotethe hexagonal close packed structure of the α-phase. The α-phase hasvery good high temperature creep properties and in near -α titaniumalloys these good creep properties are achieved while still maintainingadequate low temperature strength and forgeability.

This excellent balance of strength, ductility, microstructural stabilityand oxidation/corrosion resistance as compared with competitivematerials such as steels or nickel based superalloys has resulted innear -α titanium alloys becoming increasingly important asconstructional material for compressor components in advanced gasturbine engines.

A wide variety of these high strength near -α titanium alloys have beendeveloped commercially to tolerate operating conditions involvingprolonged exposure to air at temperatures up to 500° C. The followingcommercial alloys of IMI Titanium Ltd (identified by their commercialdesignation and nominal composition in weight percent) are typical ofthe current generation of titanium alloys suitable for gas turbinecompressor applications. These are: IMI 550 (Ti-4Al-2Sn-4 Mo-0.5Si); IMI679 (Ti-11Sn-2.25Al-5Zr-1Mo-0.25Si); IMI 685 (Ti-6Al-5Zr-0.5Mo-0.25Si)IMI 829 (Ti-5.5Al-3.5Sn-3Zr-0.25o-0.3Si) and IMI 834(Ti-5.8Al-40Sn-3.5Zr-0.7Nb-0.5Mo-0-35Si-0.06C-0.090).

There is continuing commercial interest in increasing the temperaturetolerance of near -α titanium alloys in order that gas turbine enginecycle temperatures can be raised to increase engine efficiency withoutrecourse to other materials. There is also the possibility that anincrease in temperature tolerance might permit these titanium alloys tobe used for components currently made of say nickel-based superalloyswith consequent reduction in component weight. Some of the conventionalnear -α titanium alloys exhibit excellent creep resistance and goodstructural stability at temperatures significantly above the 500° C.mentioned earlier - say to 650° C. However, when such alloys are exposedto air at temperatures approaching 600° C. they are subject tosignificant and detrimental surface modification. The principal factorinvolved in this surface modification is the uptake of oxygen into solidsolution by the titanium. At these temperatures the reaction kineticsensure rapid diffusion of oxygen into the region adjacent the exposedsurface. Approximately 30 atomic percent may be dissolved in thetitanium. The dissolved oxygen creates in the affected region a hardbrittle zone. The affected zone is called the alpha case and itsformation can substantially degrade the structural integrity of theaffected article by loss of tensile ductility and of fatigue resistanceeven though the interior of the article is not subject to structuralmodification. Nitrogen plays some part in the degradation process whichoccurs but the predominant factors in alpha case formation are thepresence of oxygen, the exposure time and the temperature. The terms"oxidation", "oxidation resistance" and related terms are hereinafterintended to encompass both oxygen and nitrogen effects; the terms beingused to distinguish these effects from other forms of attack bothchemical - such as aqueous corrosion and high temperature sulphidationor alkali metal salt corrosion and non-chemical attack by erosion.Corrosion can be a life limiting factor under some circumstances forexample gas turbine components for aircraft that fly at low altitudesacross the sea in high salt environments. It is desirable therefore,that a coating can also convey some degree of protection againstcorrosion attach.

The phenomenon of high temperature oxidation in titanium and titaniumbased alloys is not new-found and attempts have already been made toovercome this. One approach to this is to vary the alloy composition insuch a way as to convey adequate intrinsic resistance. However, allalloy compositions represent an optimisation of conflicting requirementsand it is quite likely that such titanium alloy compositions as could beproduced to solve the oxidation problem would be deficient in otherrespects. The aforementioned present generation titanium alloysespecially IMI 829, IMI 834 and other near -α titanium alloys are wellestablished in the art and would be suitable for higher temperatureapplications if adequately protected against high temperature oxidation.

Although protective coatings are well established and widely used forthe corrosion/oxidation protection of the nickel-based superalloys usedin the hot sections of the turbine stage, oxidation protective coatingsare not yet well established for titanium compressor components.

There are special difficulties involved in providing a coating that isresistant to high temperature degradation, impervious to oxygen, adheressecurely to the near -α titanium substrate and can be deposited by aprocess that does not impair the properties of the substrate.

Plasma spraying conventionally used in coating a wide variety ofmaterials such as nickel based superalloys, deposits coatings that arecharacteristically porous and internally stressed and consequentlyrequire substantial post deposition treatment to form an oxidationresistant coating. The heat treatments necessarily involved will oftenbe at temperatures that reverse properties carefully secured in the near-α titanium alloys through prior heat treatment at temperaturestypically in the region of 1000°-1100° C. We believe plasma sprayingwill be inappropriate for coating near -α titanium alloy substrates forthis reason.

It is known that surface enrichment of α/β titanium alloys with platinumimproves the oxidation resistance (as measured by fatigue properties) attemperatures up to 450° C but the published work on this does notsuggest any improvement in oxidation resistance at higher temperaturesnor is such an improvement suggested by the Applicant's experiments withplatinum enrichment. The Applicant has found also that only marginalimprovement in high temperature oxidation resistance is secured bycoating with gold, or nickel, or chromium or platinum overlaid withnickel, these being deposited by a sputter ion plating method.

Aluminide coatings and chromised coatings are well known in theprotection of nickel based superalloys. These are produced either bydeposition of pure metal by cementation or chemical vapour depositionfollowed by heat treatment to form the intermetallic compound, oralternatively by physical vapour deposition of the intermetallic. Suchcoatings are not suitable for conveying high temperature oxidationresistance to titanium or titanium alloy as they form brittleintermetallic phases at the interface with the parent material whichrenders the coating susceptible to spallation damage, particularly whensubject to thermal cycling. Moreover the deposition temperature of 750°C. or more required for cementation or chemical vapour depositionprocesses would unacceptably degrade the properties of a near -αtitanium alloy component.

Chemical routes generally such as chemical vapour deposition tend not tobe suitable for coating near -α titanium alloys, as surface cleanlinessand contamination are very important. Both halide and carbonylatmospheres used in such processes, along with the temperatures neededfor deposition make it difficult to clean the surface to preventcontaminants being introduced that can impair adhesion. Contamination atthe coating stage can also lead to the formation of an embrittled layerwithin the substrate below the coating which is detrimental to thesubstrate in itself and is detrimental also to the durability of thecoating. Halide contamination can lead to the formation of suchembrittled layers as well as oxygen contamination which is a particularproblem because of the high solubility of oxygen in titanium.

SUMMARY OF THE INVENTION

The aim of this invention is to provide an oxidation resistant coatingfor the protection of near -α titanium alloys to increase thetemperature to which such alloys suitably coated can be operational ingas turbine engine compressor applications. To achieve this aim acoating must protect the substrate against the loss of fatigueresistance, toughness and ductility due to oxidation that will preventnear -α titanium alloys being used for components with an operationaltemperature much in excess of 600° C. As recently developed near -αtitanium alloys exhibit the creep resistance and structural stabilitynecessary to withstand repeated excursions to well above 600° C.Suitable coatings to protect components such as gas turbine enginecompressor components from oxidation and /or corrosion would enable thehigh temperature mechanical properties of near -α titanium alloys to beutilised by increasing operational temperatures to well above 600° C.

The invention is an article comprising a near -α titanium based alloywhen coated with a high temperature oxidation and/or corrosion resistantcoating, the coating being applied by a process selected from the groupconsisting of ion plating, sputter plating and sputter ion plating andcomprising an iron-chromium based alloy wherein the iron and chromium incombination constitute at least 75% by weight of the applied coatingwith an iron to chromium ratio in the range 10:1 to 2.5:1 by weight.

The article may be wholly metal or a reinforced metal such as a metalmatrix composite. The coatings of this invention possess good intrinsicresistance to high temperature oxidation and low permeability to oxygenand nitrogen whilst possessing other attributes which are desirable suchas resistance to spallation when thermally cycled, and interfaceductility consequent upon the formation of ductile titanium-nickelintermetallic phases. There is some mismatch between the coefficient ofexpansion of the coating and titanium based substrates but this is notso great as to present a problem.

The coating may contain other elements in addition to the iron-chromiumbase, but preferably retains a iron to chromium ratio within theaforementioned range. To promote a film of stable aluminium oxide at theexposed surface of the coating aluminium may be included, preferably2-12 percent by weight. Silicon may also be included to provide aharder, more erosion resistant surface. A preferable range for thesilicon addition is 2-12 percent by weight. Yttrium may be includedalso, but is principally of benefit in aluminium containingcompositions, for promotion of oxide scale adherence at highertemperatures. Up to one weight percent of yttrium may be included. Thecoating may include other elements capable of contributing to theeffectiveness of the system, in minor proportion say up to one weightpercent. Elements in this class are boron, precious metals (platinumrhodium and silver), rare earths (lanthanum and cerium) and somereactive elements (hafnium titanium and zirconium). The coating mayinclude low levels of the impurities found in commercial candidatematerials--such as carbon--and may include other elements present incandidate materials which do not contribute significantly to the coatingbut cause no impairment.

The ion plating, sputter plating and sputter ion plating are allsub-atmospheric pressure process which can be performed under argoncover so they minimise the possibilities for contamination duringcoating. Moreover with each of these it is possible to pre clean thesubstrate by ion bombardment then move directly to the deposition stage.These plating processes are well established in the art so no generaldescription of them is offered here.

Specific examples of coated titanium based components according to theinvention are described below. Reference is made to uncoated titaniumbased components for the purpose of comparison. The comparison of coatedand uncoated specimens is made with reference to the accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a microsection of a coated specimen in the as-coatedcondition;

FIG. 2 is a microsection of a coated specimen which has been subjectedto cyclic oxidation testing;

FIG. 3 is a microsection of an uncoated specimen which has beensubjected to cyclic oxidation testing;

FIGS. 4-7 are graphs depicting microstructural modification consequentupon oxidation testing, as measured by microhardness measurements on atraverse across the surface regions.

DESCRIPTION OF THE PREFERRED EMBODIMENTS EXAMPLE 1

Cylindrical test pins to IMI829 alloy were coated to a depth ofapproximately 5 μm by means of a hot rod target sputtering process usingtargets of British Standard S124 stainless steel. S124 steel has thenominal composition by weight of: 12Cr-1(Mo +Zr)-0.15C-balance Fe. Thesputtering process was of the type described by R G Duckworth in ProcInt Conf on Thin and Thick Film Technology, Ausberg, 1977, NTG60, VDE,Berlin 1977 pp 83-87. There are no significant differences between thesputtering coefficients of the elements in the S124 target so thecoating deposited by the sputtering process should have the samecomposition as the target. This composition was confirmed by microprobeanalysis of the coatings.

FIG. 1 shows a microsection at ×1000 magnification of an IMI829 test pinwhich has been coated as described above. It will be seen that thecoating is dense and adherent. A traverse of microhardness measurementsacross a section as shown reveals no significant variation in hardnessas would be indicative of surface contamination in the coating process.

The S124 coated test pins were subjected to cyclic oxidation testing byexposure to air in an open furnace at a furnace temperature of 650° C.Each cycle consisted of 24 hours in furnace and 1 hour out of furnace.The test pins were weighed to assess weight gain and optically examinedto assess the coating condition. These tests were continued foraccumulated times of several hundreds of hours. Like tests were alsoperformed on further coated test pins using a more severe regime with afurnace temperature of 700° C. For each of these tests uncoated controlspecimens were also used for comparative purposes. Optical examinationof the coated specimens revealed that, providing sufficient care wastaken to ensure surface cleanliness at the coating stage, the adheranceof the coating was excellent and there was no spallation damage orcracking at the cessation of the cyclic oxidation tests. No significantweight gain was detected.

The formation of alpha case in titanium alloys is manifest as a changein microstructure and this is detectable by suitable stain etching of across-sectioned specimen. The alpha case is also detectable bymicrohardness measurements for the alpha case has a hardness in theregion of 700 on the Vickers diamond pyramid hardness scale comparedwith a base of around 400 for unaffected material.

FIG. 2 shows a microsection at ×1000 magnification of a S124 coatedspecimen which has been subjected to 150 hours of cyclic oxidationtesting using a furnace temperature of 650° C. It will be seen that thismicrosection reveals no significant change in microstructure incomparison with the pre test section shown in FIG. 1. FIG. 2 alsoconfirms that the coating remains intact and fully adherent atcompletion of the test. FIG. 3, which is a microsection taken at thesame ×1000 magnification of an uncoated specimen subjected to the sameoxidation test, shows by comparison a significant degree of alpha caseformation.

FIG. 4 compares microhardness measurements of coated and uncoatedspecimens after completion of 150 hours cyclic oxidation testing with afurnace temperature of 650° C. The uncoated specimen exhibits increasedhardness consequent upon alpha case formation to a depth of at least 60μm. For the coated specimens there is some increase of hardness relativeto the inner region but the degree of change and the depth ofpenetration are much reduced.

FIG. 5 makes a similar comparison after cyclic oxidation testing for 100hours using a furnace temperature of 700° C. It will be seen that evenat this more extreme temperature the increase of hardness and depth ofpenetration are still reduced for the S124 coated specimens to aworthwhile degree relative to the uncoated specimens through a deeperpenetration of the hardened zone is noticeable. FIGS. 4 and 5 might seemto indicate that some alpha case formation takes place despite theprotective coating. However no alpha case formation of significantextent is detectable by optical examination of microsections. Electronmicroprobe analysis of S124 coated specimens after long term oxidationtesting has revealed some interdiffusion of elements between coating andsubstrate particularly an inward diffusion of iron. This is likely toproduce a small change in the hardness of near-surface regionssufficient to produce the degree of change noticeable in FIGS. 4 and 5.However the degree of change is not likely to be such as to causeproblems of embrittlement.

EXAMPLE 2

Cylindrical test pieces of IMI829 alloy were coated using the hot rodtarget sputtering process mention previously, to a depth ofapproximately 5 pm using targets consisting of a commercial alloy havingthe composition by weight of 15.5Cr-4.8 Al-0.3Y-0.3Si-0.03C-balance Fe.No significant variation from this was identified when the compositionof the coating deposited was checked by electron microprobe analysis.Specimens thus coated have been subjected to cyclic oxidation testing ofthe type described previously, some at 650° C. furnace temperatureothers at 700° C. furnace temperature for some hundreds of hours. Atcompletion of the tests there was no coating damage by separation,spallation or cracking evident by optical examination at sectionedspecimens. Sectioned specimens were also subjected to microhardnessmeasurement on a traverse through the surface layers. The microhardnesstraverse for a specimen subjected to 400 hours oxidation testing using afurnace temperature of 650° C. is plotted in FIG. 6. A similar plot fora 200 hour 700° C. specimen is shown in FIG. 7. FIG. 6 shows noappreciable hardening consequent on alpha case formation in the 650° C.test and FIG. 7 shows there is very little hardening from the 700° C.test. There is some scatter of individual points. This is not areflection of poor measurement resolution rather the variation inmicrohardness experienced within the microstructure of normal titaniumalloy.

EXAMPLE 3

IMI829 test pins have been coated using a different sputtering processusing targets of the same commercial alloy used for Example 2. Theprocess used was a sputter ion process of the type described by J ERestall and J P Coad at page 499 et seq. in Metals Technology 9, 1982.The test pins were subjected to an alternating positive/negative biasvoltage during the deposition process for better control of coatingdistribution and microstructure. The composition of the coating waschecked as for Example 2 by electron microprobe analysis and found to besubstantially the same in composition as the target. The coatingpresented a more finely polished surface. Coating thickness varied withdeposition process duration between 10 μm and 50 μm. 10 μm is themaximum permitted by current compressor design specification to avoidany loss of compressor surge margin. Consequently the more thicklycoated specimens are unrepresentative of compressor components. The morethinly coated specimens were subjected to cyclic oxidation tests asgiven to the specimens in Examples 1 and 2. These tests weresatisfactorily completed but longer term tests, yet to be completed willbe necessary in order to give a feel for the relative merits of thecoatings of Example 3 with regard to the others given the greatercoating thickness.

We claim:
 1. An article comprising a near -α titanium based alloy whencoated with a high temperature oxidation and/or corrosion resistantcoating, the coating being applied by a process selected from the groupconsisting of ion plating, sputter plating and sputter ion plating andcomprising an iron-chromium based alloy wherein the iron and chromium incombination constitute at least 75% by weight of the applied coatingwith an iron to chromium ratio in the range 10:1 to 2.5:1.
 2. An articleas claimed in claim 1 in which the iron-chromium based coatingadditionally comprises 2-12% by weight aluminium.
 3. An article asclaimed in claim 1 in which the nickel-chromium based coatingadditionally comprises 2-12% by weight silicon.
 4. An article as claimedin claim 1 in which the iron-chromium based alloy incorporates yttrium.5. An article as claimed in claim 1 in which the iron-chromium basedalloy incorporates boron.
 6. An article as claimed in claim 1 in whichthe iron-chromium based alloy incorporates lanthanum or cerium.
 7. Anarticle as claimed in claim 1 in which the iron-chromium based alloyincorporates platinum or silver or rhodium.